Titration is one of the most selective and accurate analytical techniques available for the quantitative determination of soluble chemical compounds. In titrimetry, substances are quantified by measuring the volume of a solution with known concentration of reagent that is required for a defined, chemical conversion with the substance being analyzed. Generally, reagent, herein referred to as the “titrant”, is added to the sample until one can determine the point at which the sample is completely consumed by the titrant, herein referred to as the “end-point”, by means of a readily measurable change in a physical or chemical property at the end-point e.g. change of colour, pH, conductivity etc. Knowing the volume of sample and titrant added up to the end-point, the concentration of the titrant, and the stoichiometric relationship between the sample and the titrant the concentration of the sample can be calculated.
Classical manual titrations are carried out using a burette to accurately deliver the titrant to a known volume of sample with an indicator that undergoes an easily recognisable colour change at the end-point. Titrant is added drop-wise to the sample, mixing between additions, until a permanent colour change of the sample solution occurs, at which point the volume of titrant consumed is read from the burette. As with most analytical procedures, more than one titration is often required to reduce the chance of error. Consecutive titrations are often carried out faster by the continuous addition of titrant up to an amount just prior to the end-point, followed by precise drop-wise additions of titrant up to the end-point. The initial continuous addition of titrant up to an amount just prior to the end-point is herein referred to as “pre-titration”.
In the modern laboratory, manual tirations have largely been replaced by automated systems. Such machines help to reduce the need for laborious procedures and/or specific operator skills, and with the further implementation of robotic sample changers, increased efficiency and through-put can be achieved. Automated laboratory titrators have also been modified to monitor industrial process streams, utilizing either intermittent or continuous sampling techniques. More recently, automated titrations have been greatly miniaturized, allowing greater throughput and titrations have been demonstrated down to femtoliter volumes. However, such systems remain by nature, batch-wise, regardless of the size or throughput of the analyzer.
In order to achieve continuous titration, methods have moved to the flow domain. Generally, such methods comprise two pumps; one pumping the sample while the other delivers the titrant. Commonly, a reaction coil and/or a mixing device is employed to ensure either a partial or a complete chemical reaction between the sample and titrant prior to some form of electronic detection device. The past three decades have seen the introduction of numerous continuous flow titration devices based on this general principle, and of particular interest are the gradient methods. The gradient technique is based on the principle that the flowrate (or concentration) of either the sample or the titrant stream is continuously varied, while other parameters remain constant, providing sample concentration proportional to titrant concentration. While gradient techniques have been shown to adapt well to on-line systems, time is still required to generate a concentration gradient which results in a succession of individual results. True continuous methods should provide a readout or indication of the state of a streaming sample in real time.
According to this definition, a continuous flow titration method has been disclosed by DE-A1-2001 707 (Giacobbo and Marly-le-Grand). Said document suggests a method, where the sample to be titrated is continuously pumped through a capillary and where titrant is added at particular points along this capillary in known amounts. After each consecutive addition of titrant, a complete chemical reaction with the sample occurs until the sample is completely consumed. At the end of each reaction in the capillary, detectors are utilized to indicate the chemical status of the sample. In addition, it is suggested that a continuous pre-dilution or pre-titration is performed on the sample in order to increase the precision of the method.
However, DE-A1-2001 707 does not show any experimental results and the examples described in this document are hypothetical. Experimental work carried out to apply the principle and use of the method described by DE-A1-2001 707 lead to unexpected difficulties. It was discovered that unless all flows (sample as well as titrant) were extremely constant, fluctuating end-points and inconsistent results were observed. The majority of pumps, e.g. peristaltic or piston-driven pumps, produce small variations in flow rate that adversely affected the resulting accuracy of the titrations. DE-A1-2001 707 does not describe how to supply a continuous amount of titrant to the consecutive titrant addition points, other than it is suggested to utilize a pump that pumps the same flow for all ten addition points.